1. Field of the Invention
This invention relates to methods of forming articles consisting of particles of refractory metal carbide disposed in a metal matrix and to composite structures formed by this method.
2. Prior Art
The refractory metal carbides such as tungsten carbide, titanium carbide, tantalum carbide and the like are among the hardest man made materials and are logical choices for use as cutting tools, wear and abrasion resistant parts, etc. However, these materials are extremely brittle and will not withstand any appreciable impact forces. Accordingly, these metals have typically been used as constituents of composites in which microscopic particles of the carbides are supported in a more ductile metallic matrix. The most common method of achieving this composite form is by sintering micron size particles of refractory carbide and a powder metal binder from the cobalt, nickel, iron family. Such sintered or cemented carbides are commonly employed as cutting tools, wire drawing dies and the like. They are however relatively expensive compared with other industrial metals and too brittle for many applications where impact forces are exerted on the part.
In certain applications the high cost and brittle nature of the sintered carbides has been overcome by forming a composite consisting of sintered carbide particles or chunks supported in a matrix of a more resilient metal. The wear resistance of the sintered particles is complemented by the toughness of the matrix to form a material that is more abrasion resistant than the matrix material and can withstand impact loads better than the sintered carbide. The composites often make use of sintered carbide derived from worn cutting tools or scrap material produced during the sintering process.
Articles formed from these composite materials are used in applications where they are subjected to regular contact with hard, abrasive materials as conventional materials either wear too quickly or lack the impact resistance to withstand use over a long period. For example, they may be employed in ore treatment plants as chutes, or as facings on rock drills. They may also be employed in security applications such as for locks and safes because of their resistance to penetration by drills and like tools.
The failure mode of the composites in high abrasion typically involves the erosion of the matrix portion of a surface until a substantial portion of a sintered particle is exposed, and then either tearing away of that particle from the matrix or chipping off the exposed, brittle carbide particle. Efforts to improve the composite to minimize this failure mode have been directed toward use of harder matrix materials to minimize their erosion. But this usually increases the brittleness of the matrix making it easier for a particle to break away by cracking at the matrix-particle interface. In addition, the whole composite may become brittle as the matrix hardness is increased.
Previous efforts to form composite materials consisting of sintered metallic carbide particles in a soften metal matrix have been directed toward avoidance of any deterioration of the sintered material as a result of the heat of the molten matrix. Typically, copper based alloys have been employed for the matrix because of their low melting temperatures in the range of 1900.degree. F. to 2100.degree. F. In applications where a harder matrixing material is required, using metals with melting temperatures close to the temperature at which the metallic carbide binder melts, attempts have been made to very carefully control the temperature at which the composite is formed to minimize the amount of sintered material dissolved in the matrix. For example, U.S. Pat. Nos. 3,175,260 and 3,149,441 disclose methods wherein the steel matrix is heated to a temperature only sufficient to allow it to be poured over and infilitrate the tungsten carbide particles disposed in a mold. The particles are preheated to this infilitration temperature and the composite is maintained at this temperature for a sufficient time after pouring to insure thorough infiltation of the particle mass by the matrix material.